(1) Field of the Invention
This invention relates to the purification of fatty acid ester products.
(2) Description of the Prior Art
Sucrose esters have been prepared commercially for years by the transesterification of sugars with fatty acid esters in the presence of a basic transesterification catalyst. Reuben O. Feuge, et al, disclosed in U.S. Pat. No. 3,714,144, a process for producing sucrose esters of fatty acids having from 2 to 22 carbon atoms. The Feuge process comprises melting and mixing sucrose with esters of the fatty acids and a catalyst consisting of the alkali-free soaps of saturated or unsaturated fatty acids or mixtures thereof, reacting under vacuum to remove the alcohol or alcohol-like portions of the fatty acid esters, etc., removing the unreacted sucrose, and either removing the soap or soaps or acidifying them and removing the alkali metal portions. Use of the soaps both as catalysts and solubilizers is essential in the esterification of molten sucrose with glycerides of fatty acids, and the sugar-ester products resulting from the reaction normally contain 10% to 20% soaps.
Other processes for preparing sucrose esters of fatty acids which have been proposed result in reaction products containing 30% or more of soaps. One of these other processes was devised by Lloyd D. Osipow and William Rosenblatt (see Journal of the American Oil Chemists Society, vol. 44, pages 307-309, 1967).
The problem of economically removing unwanted soaps from sucrose-ester products is a major deterrent in the increased utilization of sucrose esters as emulsifiers. What is sorely needed is an economically feasible method of removing the soaps or their degradation products in the manufacture of sucrose-ester products. Free fatty acids in a sucrose ester product intended for use in foods are not as objectionable as soaps. Also, free fatty acids can be removed from sucrose-ester products by ultrafiltration or selective adsorption. (See H. J. Zeringue, Jr., and R. O. Feuge's article in the Journal of the American Oil Chemists Society, vol. 53, pages 567-571 and pages 719-721, 1976.)
Other processes employed in the manufacture of edible fat and oil products (glyceride esters) involve soaps and their removal. In the manufacture of monoglycerides by the interesterification of a fat or oil with glycerol it is customary to use sodium hydroxide as a catalyst, which results in the presence of about 1% soaps in the reaction product. Edible fats and oils are commonly refined by mixing with aqueous solutions of sodium hydroxide or sodium carbonate. The resulting soaps and some other impurities are removed from the oil or fat as soapstock, but the refined fat or oil still contains a minute amount of soaps which are normally removed by water washing. Disposal of this wash water without polluting the environment has been a problem. An economical process for decomposing these dilute soaps so as to permit reuse of the water and use of the products resulting from the decomposition of the soaps would be a boon to the oil and fat processing industry.
It is well known that carbon dioxide in water reacts with alkaline compounds and that sodium bicarbonate in a water solution decomposes to sodium carbonate as carbon dioxide is removed. It is also known by those skilled in the art that mixing aqueous sodium bicarbonate solution with a glyceride oil containing free fatty acids results in the formation of some soaps, but a large proportion of the free fatty acids does not react. Carbon dioxide at atmospheric pressure when added to a fatty acid ester containing soaps, even in the presence of dissolved water, will degrade the soaps only to a very limited extent, and subsequent filtration will not separate any degradation product or residual soap. As one example, Chester Placek and George W. Holman (Industrial and Engineering Chemistry, vol. 49, No. 2, February 1957, pages 162-170) state that the addition of carbon dioxide to interesterified lard to kill the alkaline catalyst resulted in the formation of soaps. The carbon dioxide and water did not decompose the soaps to form free fatty acids.
The efficacy of carbon dioxide and water in decomposing soaps differs greatly from that of relatively strong organic acids, such as citric acid, and strong inorganic acids, such as hydrochloric and sulfuric. Carbonic acid (carbon dioxide and water) is far from being equal to citric acid in decomposing soaps. The first stage of ionization of citric acid at 0.1 molar concentration is 40 times that of carbonic acid at the same concentration. But only 0.04 mole of carbon dioxide will dissolve in a liter of water at room temperature, and not all of this carbon dioxide will form carbonic acid. On the other hand, about 3 moles of citric acid will dissolve in a liter of water. Under some conditions, the addition of carbonic acid (carbon dioxide and water) to a soap solution will form bicarbonate and free fatty acids; but under other conditions, the same bicarbonate and free fatty acids react to form carbonic acid and soap. Reasonable completion of the reaction with carbonic acid in either direction depends on a complex interplay of concentrations, solubilities, reactivies, and equilibria.
In the instant invention we have found a set of conditions under which carbon dioxide in water at atmospheric pressure decomposes substantially all of the soaps present in a fatty acid ester product and permits removal of the alkali metal ions as bicarbonates and carbonates, which can be reused to form soaps in the course of other processing of fatty acid ester products.